WASHINGTON (AP) — The Supreme Court on Monday promised an extraordinarily thorough springtime review of President Barack Obama's historic health care overhaul — more than five hours of argument, unprecedented in modern times — in time for a likely ruling affecting millions of Americans just before the presidential election.

That ruling, expected before next summer's Independence Day holiday, could determine the fate of Obama's signature domestic achievement, the most far reaching domestic legislation in a generation but a political lightning rod as well. It is vigorously opposed by all of Obama's prospective GOP opponents.

The Patient Protection and Affordable Care Act aims to provide health insurance to more than 30 million previously uninsured Americans. But Republicans have branded the law unconstitutional since before Obama signed it in a ceremony in March 2010.

The court's ruling could be its most significant and political decision since George W. Bush's 2000 presidential election victory. But the justices left themselves an opening to defer the outcome if they choose, by requesting arguments on one lower court's ruling that a decision must wait until 2015, when one of the law's many provisions takes effect.

Legal experts have offered a range of opinions about what the high court might do. Many prominent Supreme Court lawyers believe the law will be upheld by a lopsided vote, with Republican and Democratic appointees ruling in its favor. But others predict a close outcome, with Justice Anthony Kennedy, a Republican who sometimes joins his four Democratic colleagues, holding the deciding vote.

The White House has pushed for a final ruling as soon as possible, and Communications Director Dan Pfeiffer said the administration was pleased the justices agreed to take the case now, with arguments in March. "It's important that we put to rest once and for all the issue of maybe the law will disappear," said Health and Human Services Secretary Kathleen Sebelius.

Republicans also said they were happy the high court would hear arguments on the constitutionality of the provision at the heart of the law and three other questions about the act. The central provision in question is the requirement that individuals buy health insurance starting in 2014 or pay a penalty.

"That the Supreme Court is taking this up, I think, is a positive signal that there are legitimate concerns surrounding the constitutional aspects of mandating that individuals purchase health care insurance and purchase it according to Washington's guidelines," said House Majority Leader Eric Cantor of Virginia.

The exceptional five and a half hours allotted for argument demonstrates the significance the justices see in this case. Normally, they allow only one hour, split between two sides. In the modern era, the last time the court increased that time anywhere near this much was in 2003 for consideration of the McCain-Feingold campaign finance overhaul. That case consumed four hours of argument.

This argument may even spread over two days, as the justices rarely hear more than two or three hours a day.

The health care overhaul would achieve its huge expansion of coverage by requiring individuals to buy health insurance starting in 2014, by expanding Medicaid and by applying other provisions, many yet to take effect.

The central question before the court is whether the government has the power to force people to buy health insurance. The White House says Congress used a "quintessential" power — its constitutional ability to regulate interstate commerce, including the health care industry — when it passed the overhaul.

But opponents of the law, and the 11th U.S. Circuit Court of Appeals in Atlanta, say that Congress overstepped its authority when lawmakers passed individual mandate. A divided Atlanta court panel ruled that Congress cannot require people to "enter into contracts with private insurance companies for the purchase of an expensive product from the time they are born until the time they die."

The Atlanta court is the only one of four appellate courts that found the mandate unconstitutional. The federal appeals court in Cincinnati upheld the entire law, as did appellate judges in Washington, D.C., in recent days. The appellate court in Richmond, Va., ruled a judicial decision on the law cannot be rendered until 2015, after the penalties for not having insurance have gone into effect.

Supporters have been encouraged that the appellate rulings in Cincinnati and Washington to uphold the law were joined by two prominent conservative judges appointed by Republican presidents: Jeffrey Sutton, appointed by George W. Bush, and Laurence Silberman, appointed by Ronald Reagan.

In Atlanta, however, Frank Hull, appointed by President Bill Clinton, joined with a Republican colleague in striking down the mandate.

Earlier District Court rulings followed political affiliation: Judges appointed by Democratic presidents upheld the law, while Republican appointees struck it down.

In addition to deciding the constitutionality of the central mandate, the justices also will determine whether the rest of the law can take effect even if that core is held unconstitutional. The law's opponents say the whole thing should fall if the individual mandate falls.

The administration counters that most of the law still could function, but says that requirements that insurers cover anyone and not set higher rates for people with pre-existing conditions are inextricably linked with the mandate and shouldn't remain in place without it.

The court also will look at the expansion of the joint federal-state Medicaid program that provides health care to poorer Americans, even though no lower court called that provision into question. Florida and 25 other states argued unsuccessfully in lower courts that the law goes too far in coercing them to participate by threatening a cutoff of federal money. The states contend that the vast Medicaid expansion and the requirement that employers offer health insurance violate the Constitution.

"The court recognized the seriousness of these vitally important constitutional challenges by allocating an extraordinary amount of time for oral argument," Florida Attorney General Pam Bondi said.

Lastly, the justices will consider whether arguments are premature because a federal law generally prohibits challenges to taxes until the taxes are paid. The federal appeals court in Richmond year reasoned that the penalty for not purchasing insurance will not be paid before federal income tax returns are due in April 2015, therefore it is too early for a court ruling.

The administration sought prompt Supreme Court review and shunned the available options for trying to delay the court's consideration until after the election. The Justice Department passed up the chance to ask the appeals court in Atlanta to reconsider its decision, though it is common to seek review by the full appeals court when a three-judge panel rules against the government.

Six separate appeals were filed with the high court. Three came from the Atlanta court, where the administration, the states and the National Federation of Independent Business appealed different aspects. From Richmond, Liberty University and Virginia appealed court decisions turning back their challenges. The Thomas More Law Center of Ann Arbor, Mich., appealed a ruling by the Cincinnati-based court upholding the law.

Ultimately, the court chose the Atlanta court's ruling as the primary case to review. That decision means that a highly regarded former Bush administration solicitor general, Paul Clement, is likely to argue on behalf of the challengers. The current solicitor general, Donald Verrilli Jr., is expected to defend the law before the justices.

Two justices, conservative Clarence Thomas and liberal Elena Kagan, who had been asked by advocacy groups to withdraw from the case, are going to take part in it. The court's practice is for justices who are staying out of a case to say so when it is accepted, and no one has announced a recusal. Thomas's wife, Virginia, has worked for a group that has advocated against the health care overhaul, and Kagan served as solicitor general in the Obama administration when the law was being formulated.